<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div {
            width: 200px;
            height: 200px;
            background: red;
            color: #ffffff;
        }

        .div1 {
            width: 150px;
            background: green;
            margin-top: -50px;
        }

        span {
            display: block;
            height: 100px;
            width: 100px;
            background: blue;
            color: #ffffff;
            margin-top: -50px;
        }
    </style>
</head>
<body>
<!--
       在正常情况下，层级的大小由顺序决定，后面的元素比前面的元素层级要高
-->
<div>div1</div>
<div class="div1">div2</div>
<span>span</span>
</body>
</html>